Silicone implants from the 1980's?

do not need to be replaced if you are happy with how things are for you.  If you look with an MRI and your implant is ruptured, the recommendation will be for replacement.  So don't look unless you want to consider replacing them.  The capsule that formed around your original implant will contain any silicone unless you are involved in some high impact trauma to your chest.

Silicone leak (except from the recently recalled PIP implants) is not dangerous; silicone is used in joint replacements and other medical devices.

Breast implants from the 1980s had thinner shells and much more liquid silicone than is used today.

The silicone used in implants now is much firmer; if you cut through the centre of an implant today, it looks like cutting through a "jelly baby").

If the silicone leak is contained within the scar tissue that forms around the breast implant (capsule) it just remains adjacent to the original implant. If the silicone leak migrates past this (less commonly), it can end up in lymph glands in the armpit or in the surrounding tissues such as muscle. In these cases, it can cause cause symptoms (such as swelling in the armpit glands) and discomfort as well as a poor appearance. Whilst this is not dangerous, it may be uncomfortable.

All implants I have removed from the 1980s have some degree of leak, usually it is contained, sometimes it is in the surrounding tissues (making surgical removal more challenging).

Once again to emphasise it is not dangerous, however, many people do not like the appearance of their implant. 1980s implants had a much higher rate of capsular contracture (scar tissue around the implant causing it to feel firm and look distorted) than the implants we use today.

I would say implants, even today with more durable shells and firmer silicone, are not a lifetime device. Exchange, however, is usually more for cosmetic appearance rather than safety.

If your implants look great, and don't bother you, I would leave them. I suspect if you do get an MRI, there will almost definitely be some type of leak from such an old implant, however, it is not unsafe to leave them alone.